1. Field of the Invention
The present invention relates, in general, to a low noise refrigerator equipped with a silencing system adopting a so-called active control method.
2. Description of the Related Art
Recently, attempts have been made to lower the noise produced by the compressor and fan motor of a refrigerator which constitute the principal sources of refrigerator noise. Progress has been made with anti-vibration designs for the refrigerant piping within the machinery chamber that accommodates the compressor. Also, by use of sound absorbing and sound insulating materials or mufflers, reduction of the high frequency components of compressor noise has been achieved to some degree. However, there is a problem that sufficient noise reduction can not be achieved by these conventional techniques in the low frequency noise band in particular.
Therefore, the inventors of the present invention have studied the application of a silencing system adopting a so-called active control method to refrigerators. In an active controlled silencing system, noise is cancelled by actively emitting a controlled sound from, for example a speaker. The noise source is detected by using a microphone such as described in U.S. Pat. No. 2,043,416. Japanese Patent Disclosure (Kokai) No. 63-311397 discloses that at least a section of the sound wave propagation path, where the silencing system is located, is constructed of a special material such as a vibration stopper or vibration absorbent. One example of the application of an active control silencing system to a refrigerator is shown in FIG. 8. The contents of FIG. 8 are presented for explanation, not as a description of the prior art. In FIG. 8, a compressor 20 is arranged in a machine chamber 10 that is located at the lowest part at the back face of the refrigerator. The compressor 20 is the main source of refrigerator noise. The machine chamber 10 has a one-dimensional duct construction, being completely sealed except for a single opening 17 for heat radiation and evaporation of defrosting water. That is, by making the dimensions of the cross-section of the duct sufficiently small in comparison with the wavelength of the compressor noise S that is to be reduced, the compressor noise S in the machine chamber 10 can be made to be a one-dimensional plane-progressive wave. The compressor noise S is detected by a microphone 35 that is arranged in a position within the machine chamber 10 remote from the opening 17. The compressor noise, i.e., the sound M that is detected by the microphone 35 is processed by a control circuit 40 of transfer function G. Circuit 40 is equipped with a finite impulse response filter (hereafter, FIR filter) that for example, directly processes the detected signal in the time domain, before supplying a compressor noise cancellation signal to the speaker 50. The compressor noise that tries to get out from the opening 17 of the machine chamber 10 is canceled by the controlled sound A produced by the speaker 50.
The transfer function G of the control circuit 40 is determined as follows. The detected sound M obtained by the microphone 35 can be represented by equation (1) below, in terms of the noise S emitted from the compressor 20 and the controlled sound A emitted from the silencing speaker 50, using the sound transfer function G.sub.SM between the compressor and the microphone and the sound transfer function G.sub.AM between the speaker and the microphone. EQU M=S.times.G.sub.SM +A.times.G.sub.AM ( 1)
For test purposes, a microphone 55 for evaluation of the silencing effect is provided at the opening 17 of the machine chamber 10. The measured sound R of the evaluation microphone 55 can be expressed by equation (2) below, using the sound transfer function G.sub.SR between the compressor and the opening, and the sound transfer function G.sub.AR between the speaker and the opening. EQU R=S.times.G.sub.SR +A.times.G.sub.AR ( 2)
Since G is the transfer function between the microphone and the speaker, the following equation (3) holds. EQU A=M.times.G (3)
In order to cancel the compressor noise that tries to issue from the opening 17, the following equation (4) should be hold. EQU R=0 (4)
From above equations (1) to (4), the transfer function G for silencing is expressed by the following equation (5). EQU G=G.sub.SR /(G.sub.SR .times.G.sub.AM -G.sub.SM .times.G.sub.AR) (5)
If the numerator and the denominator of the equation (5) is divided by G.sub.SM, the following equation (6) is obtained. G.sub.MR is defined by equation (7). EQU G=G.sub.MR /(G.sub.MR .times.G.sub.AM -G.sub.AR) (6) EQU G.sub.MR =G.sub.SR /G.sub.SM ( 7)
By using these equations (6) and (7), even if the compressor noise S is unknown, the transfer function G to make the measured sound R zero can be found by measuring the transfer function ratio G.sub.MR between G.sub.SR and G.sub.SM. On this occasion, in the condition in which noise S is generated from the compressor 20, the detected sound may be treated as an input signal and the measured sound R may be treated as a response signal.
If a transfer function G determined as above is supplied to control circuit 40, a controlled sound A corresponding to compressor noise S is generated and the noise S is canceled at the opening 17 of the machine chamber 10.
However, when the compressor noise S is detected by the microphone 35, the following problems occur. First of all, since not only the noise S from the compressor 20 but also the controlled sound A from silencing speaker 50 is picked up by the microphone 35, howling can occur. Therefore, the output of the speaker 50 must be kept fairly low, resulting in an inadequate silencing effect. An echo canceler can be fitted to control circuit 40 to prevent howling, but this raises the cost of the system. Also, if a fan for cooling the compressor 20 is provided in machine chamber 10, the noise generated by the fan will also be picked up by the microphone 35, making the control for silencing more complicated. Furthermore, there is a risk that the silencing system would react to, for example, an external noise.